A Mechanism for the major disruption in tokamak is proposed as follows ; that is, the m=2;n=1 tearing mode non-linearly destabilizes the other tearing modes and strongly interacts with the limiter. With flat current density profile, the m=2;n =1 tearing mode is unstable during the major disruption.
For the resistivity η(r) - $1/J_{\zeta o}(r)$, the deformation of the torioidal current density in the three dimensional case is more severe than that in two dimensional case. The plasma extends across the plasma cross section, and the plasma core shifts in the major radius. The numerical results for the major disruption time of the initial value problem throughout the three dimensional nonlinear code run are about 30-80μsec, which agrees reasonably well with the experimental value of about 100μsec.